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Science lesson plan · 7th grade (Year 8 in England) · 45 minutes

Kilowatt-hours and the cost of electricity

Energy

CCSS NGSS.MS-PS3-5CCSS NGSS.SEP 5England: KS3 Physics: Energy (energy costs)

Objective and success criteria

Students will: Kilowatt-hours and the cost of electricity.

  • I can answer questions on today's skill on my own, without help.
  • I can explain each step and say why it works.
  • I can check my answer and spot a mistake.

Before and after

Needs first: Identify energy stores in everyday situations

Leads to: the next skill in this area.

Materials: Wind-up toys or a pendulum, Energy transfer diagrams, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).

Warm-up · 5 min

  • Quick retrieval from earlier learning (Identify energy stores in everyday situations). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. Which energy store is the main one here: wood for a campfire? (elastic potential store / chemical store / kinetic store / gravitational potential store) → chemical storePrompt: Is it moving, high up, stretched, hot, a fuel or food, or atomic?
  2. An electric car's battery runs down as it drives up a hill. Which stores gain energy? (no store gains energy / the car's gravitational potential store and the thermal store of the surroundings / the car's nuclear store / only the battery's chemical store) → the car's gravitational potential store and the thermal store of the surroundingsPrompt: The car gets higher, and the motor gets warm.
  3. Energy stores: which statement is NOT true? (Energy is measured in joules (J). / Energy is used up when a battery goes flat. / Food contains energy in a chemical store. / A faster object has more energy in its kinetic store.) → Energy is used up when a battery goes flat.Prompt: Is light a store or a way of transferring energy?

Hook · 5 min

  • Wind up a toy or stretch an elastic band and ask where the energy goes.
  • Share the objective: Kilowatt-hours and the cost of electricity. Read the success criteria together.

I do: model it · 10 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: An oven with a power of 1.5 kW is used for 6 hours. How much energy does it transfer, in kWh?
  • Step 1: 1.5 × 6 = 9
  • Step 2: So the answer is: 9 kWh
  • Answer: 9 kWh
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 10 min

  • Do these together. Students try each step on whiteboards before you reveal it.
  • Use the hints as prompts rather than giving the answer.
  1. A games console with a power of 1.5 kW is used for 3 hours. How much energy does it transfer, in kWh? → 4.5 kWhPrompt: energy (kWh) = power (kW) × time (h)
  2. Electricity costs 15p per kWh. How much does 2 kWh cost, in pence? → 30Prompt: cost = energy used (kWh) × price per kWh

You do: independent practice · 10 min

  • Students work on the student sheet. Circulate and check the first two answers of every student early.
  • Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

Exit ticket · 5 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. An oven with a power of 2 kW is used for 5 hours. How much energy does it transfer, in kWh? → 10 kWhPrompt: energy (kWh) = power (kW) × time (h)
  2. Electricity costs 20¢ per kWh. How much does 12 kWh cost, in cents? → 240Prompt: cost = energy used (kWh) × price per kWh

Watch out for

  • Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
  • Remembering facts without the reason: ask "why?" and "what is the evidence?"

Support and stretch

Support

  • Review first: Identify energy stores in everyday situations.
  • Use concrete materials: wind-up toys or a pendulum.
  • Give the first step of each question, or a partly worked example to finish.
  • Pair with a confident partner for the guided practice.

Stretch

  • Explain a common mistake in writing and how to fix it.
  • Write a word problem that uses today's skill, then solve a partner's.

Homework and more practice

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-7.

Free lesson plan from eurekami.com/lesson-plan-builder

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Kilowatt-hours and the cost of electricity

Name

Date

Score/ 8

  1. Question 1

    An electric heater rated at 3000 W runs for 15 minutes. Electricity costs 24¢ per kWh. What does it cost, in cents?

    Answer

  2. Question 2

    Electricity costs 35c per kWh. How much does 20 kWh cost, in euro cents?

    Answer

  3. Question 3

    A games console with a power of 0.5 kW is used for 5 hours. How much energy does it transfer, in kWh?

    AnswerkWh

  4. Question 4

    An electric heater rated at 2000 W runs for 120 minutes. Electricity costs 20¢ per kWh. What does it cost, in cents?

    Answer

  5. Question 5

    Electricity costs 30¢ per kWh. How much does 2 kWh cost, in cents?

    Answer

  6. Question 6

    An oven with a power of 0.5 kW is used for 0.5 hours. How much energy does it transfer, in kWh?

    AnswerkWh

  7. Question 7

    An electric heater rated at 1500 W runs for 30 minutes. Electricity costs 30p per kWh. What does it cost, in pence?

    Answer

  8. Question 8

    Electricity costs 20p per kWh. How much does 3 kWh cost, in pence?

    Answer

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Answer key: Kilowatt-hours and the cost of electricity

  1. 1. 18

  2. 2. 700

  3. 3. 2.5 kWh

  4. 4. 80

  5. 5. 60

  6. 6. 0.25 kWh

  7. 7. 22.5

  8. 8. 60